The band of a transmit signal continues to increase as the speed at which radio communication is performed increases. Therefore, in a radio communication device, a high linearity is asked for a transmission unit in order to suppress degradation of the signal quality. However, in many cases, the linearity and the power efficiency of a power amplifier used in a transmission unit as mentioned above is incompatible. Therefore, in order to linearly amplify a wide band signal which is high in terms of peak-to-average power ratio (hereinafter, referred to as PAPR), it may be necessary to operate the power amplifier in a state that sufficient back-off is maintained and hence the efficiency of the transmission unit may be reduced accordingly.
As means for solving the above mentioned problem, an envelope tracking (hereinafter, referred to as an ET) system and an envelope elimination and restoration (hereinafter, referred to as an EER) system configured to control a drain voltage (or a collector voltage) of a power amplifier in accordance with a fluctuation in envelope of a transmit signal are proposed.
FIGS. 1 and 2 are diagrams illustrating a configuration of an existing ET system and the operational principle thereof. In the ET system, an envelope which has been extracted from a transmit signal using an envelope extraction unit is input into a variable power source via a digital-to-analog converter as a voltage control signal which is generated using a voltage control signal generation unit to be used to control the drain voltage of the power amplifier. Instead of applying a “fixed biased voltage (FIG. 2)” in order to output constant peak power, a “drain voltage in an envelope tracking operation (FIG. 2)” is applied to the power amplifier to output variable signal power. According to this operation, if the power amplifier is normally operated in a nearly saturated region, power consumption is “reduced by performing an envelope tracking operation (FIG. 2)”. Therefore, the power efficiency may be improved. In addition, in the ET system, it is preferable to generate a voltage control signal with which the efficiency is maximized while maintaining an out-of-band distortion level which is lower than a standard value.
A configuration which includes a voltage control signal generation unit contents of which may be adaptively updated is proposed so as to cope with a variation among power amplifiers and a fluctuation in characteristic of each power amplifier in an operation. In a technique used in the above mentioned configuration and others, the voltage control signal is generated using a look-up table and polynomial series and the contents thereof is updated in accordance with an adaptive algorithm. In addition, in a technique for controlling the impedance of an input/output matching circuit in accordance with an envelope of a transmit signal, use of polynomials in generation of a control signal of the input/output matching circuit is proposed. However, use of the look-up table may lead to an increase in necessary memory amount.